1. Field of the Invention
This invention relates to a method for preparing a test gas stream for a gas analyzer, and more particularly to a method for preparing a test gas stream for a gas analyzer in which a test gas is drawn out of an exhaust stream by a test gas probe, fed through a test gas tube to a condensate separator, and then analyzed by a gas analyzer.
2. Description of the Related Art
To analyze flue gases from a furnace or exhaust from an engine such as a gas engine, a test gas stream is tapped off the main exhaust stream by a test gas probe and conducted through a test gas tube to a condensate separator prior to analysis in the gas analyzer. Analysis of the gas is then performed either directly or with the interposition of another, longer lead, located downstream from the condensate separator. Gas analysis requires that the test gas be prepared, especially to remove the moisture contained in the gas.
During the transport of the test gas from the probe to the condensate separator, there is the danger that, because of the effects of cooling, the condensate will precipitate prematurely on the inside wall of the test gas tube, and consequently more test gas will come in contact with the liquid condensate. When this happens, gas components such as NO.sub.2 and SO.sub.2 may become partially bonded in the condensate thereby causing the concentration of these components to change as the gas flows through the test gas tube. This variation can affect the value of these gas components measured by the analyzer and the resulting accuracy of the analyzer, since the measured concentration may differ from the concentration actually present in the exhaust of the flue gas.
To avoid such measurement errors, it is known, for example from German Patent Application 42 16 404 A1, to heat the test gas tube. By heating the test gas tube, the test gas may be maintained at a temperature above the dew point as it flows through the device to prevent premature precipitation of the condensate.
In these devices, condensate separation takes place completely in the condensate separator located downstream from the test gas tube. By limiting the condensation region, it is possible to ensure that the condensate can precipitate rapidly along a short distance in the gas flow path so that the period of time during which the test gas comes in contact with the liquid condensate is limited. Components such as NO.sub.2 and SO.sub.2 are thus given only limited opportunity to bond with the condensate so that minimal distortion of the measurement results occurs.
Although this type of gas preparation has proven successful in theory, there are problems in practice, particularly in that a relatively large amount of energy must be expended to heat the test gas tube, especially since the tube may extend over a relatively long distance. In addition, the condensate separator located downstream must be subjected to considerable cooling because the test gas must be cooled in the condensate chamber to a temperature below the dew point. Since cooling to avoid measurement error is intended to take place over a distance that is as short as possible, the cooling power that must be provided is very high. This high energy consumption is particularly problematic in portable and mobile gas analyzers designed for use by chimney sweeps and furnace technicians, since portable gas analyzers typically run on batteries. The duration during which the gas analyzer can be operated is therefore very limited and spare batteries must be carried if the measuring tasks that occur during a conventional workday are to be performed.
Attempts to overcome this problem have previously been directed toward reducing the amount of energy required to heat the test gas tube. These efforts have typically involved providing the test gas tube with an insulating jacket to minimize heat loss to the environment. Unfortunately, providing an insulating jacket of this nature results in test gas tubes which are relatively heavy and awkward to handle.